Vasopressin Vs Terlipressin in Treatment of Refractory Shock

Arginine vasopressin (AVP) and its synthetic, long-acting analog terlipressin (TP) are potent alternative vasoconstrictors in the treatment of septic patients with catecholamine-refractive vasodilatatory shock.Recent clinical data suggest that early administration of AVP analogues may be advantageous compared to a last resort therapy. However, it is still unknown whether vasopressin and terlipressin are equally effective for hemodynamic support in shock. Despite important pharmacologicaldifferences between thetwo drugs the use of eithersubstance is determined mainly bylocal availability and institutional inventory. The current literature suggests that neither AVP nor TP should be administered in high doses in shock. Furthermore, increasing evidence indicates that early administration of terlipressin may improve outcome as compared to a last-resort treatment. Low-dose infusion of AVP has been demonstrated to be a safe adjunct in the management of refractory shock. Evidence from experimental studies and initial clinical reports suggests that continuous low-dose infusion of TP may stabilize hemodynamics in shock. In this review we briefly described differences in pharmacokinetics and pharmacodynamics between AVP and Terlipressin (TP) in treatment of refractory shock. I. VASOPRESSIN IN SHOCK Vasopressin (AVP) is a polypeptide with a disulphide bond between the two cysteine amino acids [1]. In humans AVP is encoded by the mRNA for preproneurophysin II. After cleavage of the signal peptide, the resulting prohormone contains AVP (nine amino acids), neurophysin II (95 amino acids) and a glycopeptide (39 amino acids). The prohormone is synthesized of the supraoptic and paraventricular nuclei of the hypothalamus. The final hormone is transported by the neurones of the hypothalamo-neurohypophyseal bundle of the pituitary gland to the secretion site, namely the posterior hypophysis. It is then stored in granule form. Of the total stock of vasopressin, 10-20% can be rapidly released into the bloodstream [2]. Secretion diminishes if the stimulus continues. This kinetic action explains the biphasic course of vasopressin plasma concentrations during septic shock, with an early elevation followed by subsequent diminution [3]. Vasopressin secretion is complex and depends upon plasma osmolarity and blood volume. The central osmoreceptors that regulate vasopressin secretion are located near to the supraoptic nucleus in the anterolateral hypothalamus in a region with no blood-brain barrier.[4]. There are also peropheralosmoreceptors at he level of the hepatic portal vein that detect early the osmotic impact of ingestion of foods and fluids. The afferent pathways reach the magnocellularneurones of the hypothalamus via the vagal nerve. These neurones are depolarized by hypertonic conditions and hyperpolarized by hypotonic …


I. VASOPRESSIN IN SHOCK
Vasopressin (AVP) is a polypeptide with a disulphide bond between the two cysteine amino acids [1].
In humans AVP is encoded by the mRNA for preproneurophysin II. After cleavage of the signal peptide, the resulting prohormone contains AVP (nine amino acids), neurophysin II (95 amino acids) and a glycopeptide (39 amino acids). The prohormone is synthesized of the supraoptic and paraventricular nuclei of the hypothalamus. The final hormone is transported by the neurones of the hypothalamo-neurohypophyseal bundle of the pituitary gland to the secretion site, namely the posterior hypophysis. It is then stored in granule form. Of the total stock of vasopressin, 10-20% can be rapidly released into the bloodstream [2]. Secretion diminishes if the stimulus continues. This kinetic action explains the biphasic course of vasopressin plasma concentrations during septic shock, with an early elevation followed by subsequent diminution [3].
Vasopressin secretion is complex and depends upon plasma osmolarity and blood volume. The central osmoreceptors that regulate vasopressin secretion are located near to the supraoptic nucleus in the anterolateral hypothalamus in a region with no blood-brain barrier. [4]. There are also peropheralosmoreceptors at he level of the hepatic portal vein that detect early the osmotic impact of ingestion of foods and fluids. The afferent pathways reach the magnocellularneurones of the hypothalamus via the vagal nerve. These neurones are depolarized by hypertonic conditions and hyperpolarized by hypotonic conditions [5].
In contrast to osmotic stimulation, arterial hypotension and hypovolaemia stimulate vasopressin exponentially. Arterial hypotension is the principal stimulus for vasopressin secretion via arterial baroreceptors located in the aortic arch and the carotid sinus [6]. It is transported by the vagal and glossopharyngeal nerves toward the nucleus tractussolitarus and then toward the supraoptic and paraventricular nuclei. Inhibition of this secretion is principally linked to volume receptors located in the cardiac cavities [7]. In a physiological situation, inhibition is constant because of diminishes then vasopressin secretion increases [8]. If central venous pressure diminishes, then these receptors first stimulate secretion of natriuretic factor, the sympathetic system, and renin secretion. Vasopressin is secreted when arterial pressure falls to the point that it can no longer be compensated for by the predominant action of the vascular baroreceptors [9][10][11].
Other stimuli can favour secretion of vasopressin. These include hypercapnia,hypoxia, hyperthermia, pain, nausea, morphine and nicotine [12]. At the hormone level, numerous molecules are direct stimulators, including acetylcholine, histamine, nicotine, angiotensin II, prostaglandins, dopamine and, especially, the adrenergic system [13]. Noradrenaline (norepinephrine) has a complex effect on vasopressin secretion [12]. At low concentrations it increases activity. At high concentrations it inhibits the production of vasopressin [14]. Nitric oxide (NO), through cGMP, is a powerful neurohormonal inhibitor of vasopressin. This pathway is of fundamental importance in the case of septic shock [15,16]. Opiates, alcohol, γ-aminobutyric acid, and Università degli Studi di Salerno auricular natriuretic factor are also inhibitors. Vasopressin acts through several receptors, These receptors are different from those of catecholamines. Vasopressin has a direct vasoconstrictor effect on systemic vascular smooth muscle via V1receptors. The same type of receptor was found on platelets, which are another storage location for vasopressin [17,18].
The V2 receptors in the renal collecting tubule are responsible for regulating osmolarity and blood volume. At certain concentrations, vasopressin provokes vasodilatation in some vascular regions. Vasopressin also acts as a neurotransmitter. The vasoconstrictor activity of vasopressin, which is mediated by the receptors, is intense in vitro. There is also a V1 probable indirect action on vascular smooth muscle cells by local inhibition of NO production [19]. However, under physiological conditions, vasopressin has only a minor effect on arterial pressure [20]. One experimental hypothesis is that the vasopressor effect of vasopressin is secondary to its capacity to inhibit smooth muscle cell K+-ATP channels [21].
This moderate effect observed in vivo can be explained by the indirect bradycardic effect resulting from vasopressin's action on baroreflexes [22]. This effect on baroreflexes is mediated by the cerebral V1receptors [23]. It requires integrity of the cardiac baroreflexes because it disappears after administration of a ganglioplegic agent. Vasopressin concentrations of approximately 50 pg/ml are required before any significant modification becomes apparent [24,25].
In shock the haemodynamic response to vasopressin becomes important in maintaining arterial pressure and tissue perfusion. Administration of V1 receptor antagonists in animals in haemorrhagic shock increases hypotension [26].
Vasopressin concentrations increase during the initial phase of shock. Thus, contrary to what is observed under physiological conditions, when the autonomous nervous system is deficient and baroreflexes altered the vasopressor effect becomes predominant and prevents severe hypotension [27]. However, its trigger differs from that of catechol-amines on several levels. Vasopressin provokes a reduction in cardiac output and its vasoconstrictor activity is heterogeneous on a topographical level [28]. Its administration provokes vasoconstriction in skin, skeletal muscle, adipose tissue, pancreas and thyroid [26]. This vasoconstriction is less apparent in the mesenteric, coronary and cerebral territories under physiological conditions [29]. Its impact on digestive perfusion is under debate. Two studies conducted in patients with septic shock [30,31] demonstrated absence of impact of vasopressin on splanchnic circulation. In contrast, in a recent study conducted in animals in a state of endotoxaemic shock [32], a reduction in digestive perfusion with vasopressinadministration was observed. Finally, contrary to catechol-amines, whose effect can only be additive, vasopressin potentiatesthe contractile effect of other vasopressor agents [33].
The vasodilatation of certain vascular regions with vasopressin is an further major difference from catecholamines. This effect occurs at very low concentrations [34]. The literature is limited on this subject. Animal studies have been reported, butthey were not conducted in the context of sepsis. Some authors reported vasodilatation at a cerebral level in response to vasopressin, with more marked sensitivity to vasopressin in the circle of Willis [32]. The mechanism of this vasodilatationcan be explained by production of NO at the level of the endothelial cells. The receptors involved have not beenclearly identified.It has been shown that vasopressin provokes vasodilatation of the pulmonary artery both under physiological and hypoxicconditions [35]. The V1 receptors are involved and cause endothelial liberation of NO [36].
The renal effect of vasopressin is complex. In response to blood hyperosmolarity it reduces urine output through its action on the V2receptors, which induce reabsorption of water. Inversely, it has diuretic properties in case of septic shock and congestive heart failure [37]. The mechanisms involved in the reestablishment of diuresis are poorly understood. The principal hypothetical mechanisms are a counter-regulation of the V2 receptors and selectivevasodilatation of the afferent arteriole (under the action of NO) in contrast to vasoconstriction of the efferent arteriole [38].
Patel and coworkers [31] recently reported a randomized study in which there were significant improvements in diuresis and creatinine clearance in patients with septic shock under vasopressin treatment as compared with patients treated with noradrenaline. It has been shown in nonseptic rats that elevated concentrations of this hormone provoked a dose-dependent fall in renal blood output, glomerular filtration, and natriuresis [39]. All of the investigators who found a beneficial effect following treatment with vasopressin for septic shock used minimal doses, allowing for readjustment to achieve physiological concentrations.
Vasopressin acts on the corticotrophic axis by potentiating the effect of the corticotrophin-releasing hormone on the hypophyseal production of adrenocorticotrophic hormone [40]. The ultimate effect is an elevation in cortisolaemia [41], which is of interest in the case of septic shock because cortisol levels can be lowered.
At a supraphysiological dose, vasopressin acts as a platelet-aggregating agent [42]. The coagulation problems in septic shock make this effect undesirable. However, the doses used are unlikely to provoke a significant aggregation effect.

II. TERLIPRESSIN IN SHOCK
Terlipressin (TP) is a synthetic analogue of AVP characterized by greater selectivity for the V1 receptor than AVP [43]. The vasopressor (V1 receptor-mediated) to antidiuretic (V2 receptor-mediated) ratios of AVP and TP are 1 and 2.2, respectively (Fig.1). [44]. The Università degli Studi di Salerno elimination half-life of TP is longer than that of AVP (50 vs. 6 min) [45]. As a prodrug, TP is cleaved by endopeptidases, resulting in retarded release of the active metabolite lysine vasopressin (LVP). Data on TP plasma concentrations after bolus injection of the drug are limited, and unfortunately no data on plasma levels after continuous TP infusion are currently available. Following bolus injection of 10 μg/kg TP in 14 healthy volunteers (equivalent to 0.7mg in a 70-kg subject), Nilsson et al. reported a peak plasma level of approx. 52,000 pg/ml within 5 min and a decline to approx. 2,750 pg/ml within 1 hour after administration. Notably, the very high TP peak plasma levels measured at this time point may have occurred because levels were determined during the distribution half-life of the drug. With respect to the different preparations (prodrug vs. active agent) the pharmacokinetics of TP and AVP are difficult to compare. Following enzyme kinetics the organism degrades TP into diglycyl-, monoglycyl-lysine vasopressin, and LVP. In the same study Nilsson et al. determined LVP plasma levels after bolus injection of 5 μg/kg TP (equivalent to 0.35 mg in a 70-kg subject). Peak LVP plasma concentrations were detected 60 min after TP bolus and averaged 106 pg/ml. Thus plasma concentrations of the main bioactive component LVP after 5 μg/kg TP bolus injection (i.e., 106 pg/ml) are comparable to plasma levels reached by continuous infusion of 1.8-2.4U/h AVP (i.e., 100-300 pg/ml).
Forsling et al. [46] reported that LVP plasma levels in healthy humans increased 40-60 min after intravenous bolus administration of 7.5 μg/kg TP and reached its peak after 60-120 min. Whereas the onset of antidiuretic effects after TP infusion were observed after approx. 120 min, the onset of vasopressor effects was detected after only 3 min. These observations indicate that the renal V2 receptor-mediated effects after TP injection are dependent on the release of LVP, while TP exerts intrinsic effects on V1 receptors. The fact that the effective half-life of TP is markedly longer than that of AVP (4-6 h vs. 6-20 min) provides a theoretical rationale for administering TP as intermittent bolus infusion of 0.5-1 mg (-2) in patients with septicshock [47][48][49][50].
The first clinical trial of the efficacy of terlipressin in septic shock was performed in a small case series of eight patients [50]. Terlipressin was administered as a single bolus of 1 mg (the dosage used in gastroenterological practice) in patients with septic shock refractory to catecholamine-hydrocortisone-methylene blue. A significant improvement in blood pressure was obtained in these patients during the first 5 hours. Cardiac output was reduced, which might have impaired oxygendelivery. Partial or total weaning from catecholamines was possible. No other side effect was observed.
Another study was conducted in 15 patients with catecholamine-dependent septic shock (noradrenaline ≥ 0.6 μg/kg per min). An intravenous bolus of 1 mg terlipressin was followed by an increase in MAP and a significant decrease in cardiac index. Oxygen delivery and consumption were significantly decreased [48].
Gastric mucosal perfusion was evaluated by laser Doppler flowmetry and was increased after terlipressin injection. The ratio between gastric mucosal perfusion and systematic oxygen delivery was also significantly improved after terlipressin injection. These findings could be related to a positive redistribution effect of cardiac output on hepatosplanchnic circulation, with an increase in blood flow to the mucosa. The adverse effects of terlipressin on oxygen metabolism were also emphazised in an experimental study conducted in sheep [51]. Terlipressin was given by continuous infusion (10-40 mg/kg per hour) and was responsible for a significant decrease in cardiac index and oxygen delivery. Oxygen consumption decreased whereas oxygen extraction increased. These modifications may carry a risk for tissue hypoxia, expecially in septic states in which oxygen demand is typically incrased.
Terlipressin was also used in children [52] in a short case series of four patients with catecholamine-resistant shock. MAP increased, allowing reduction or withdrawal of noradrenaline. Two children died.

III. SUMMARY AND CONCLUSIONS
Both AVP and TP have been proven effective in restoring sepsis-related arterial hypotension and reducing catecholamine requirements in the experimental [53][54][55] and clinical [56,48,49,30] setting. In high doses, however, administration of either compound may be associated with adverse effects, basically related to excessive systemic and/or regional vasoconstriction, leading to a reduction in cardiac output and systemic oxygen delivery [30,49], impairment of intestinal microcirculation [48,57], increase in pulmonary vascular resistance [58,59], ischemic skin lesions [60], or elevated surrogate Università degli Studi di Salerno parameters of liver injury [61,62].
The higher V1 receptor selectivity of TP towards AVP may be more potent in restoring refractory hypotension related to septic shock. The longer effective half-life of TP than AVP may help avoiding rebound hypotension after discontinuation of the drug [63] but, on the other hand, carries the risk of excessive vasoconstriction after bolus injection [64]. Results from experimental studies and clinical case reports suggest that continuous infusion of low-dose TP [65] may stabilize hemodynamics in shock with reduced side effects towards injections of 1 mg (-2) boli [60].
However, future randomized controlled clinical trials are warranted to elucidate the significance of continuous low-dose and bolus TP infusion vs. infusion of AVP in patients with shock.
Due to the lack of evidence from comparative studies, bolus or continuous administration of TP should currently be limited to controlled clinical trials.